1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a WiMAX (Worldwide Interoperability for Microwave Access) [IEEE 802.16a] system, and a wireless terminal and a wireless base station used for the system. WiMAX® is one of standards of radio communication technology, and is a generic term for technology that is standardized by the IEEE 802.16 working group and the WiMAX Forum® which is an industry organization.
2. Description of the Related Art
In a current WiMAX system, a communication area is smaller in Up Link communication than in Down Link communication. This is caused by problems of asymmetry of transmission powers or transmission and reception timing between a WiMAX base station and a WiMAX-embedded wireless terminal.
As shown in FIG. 1, a WiMAX base station (BS) receives a CDMA Ranging Request from a WiMAX-embedded wireless terminal (MS), and notifies WiMAX-embedded wireless terminal 4 of respective adjusted values of power, timing and frequency in a Ranging Response. When the respective parameters of power, timing and frequency are values within permissible ranges, the WiMAX base station notifies the WiMAX-embedded wireless terminal of Ranging completion in a Ranging Response.
Until the WiMAX-embedded wireless terminal is notified of the Ranging completion by the WiMAX base station, the terminal repeats transmission of a CDMA Ranging Request to the WiMAX base station for a prescribed number of times by referring to adjusted values notified in the Ranging Response by the WiMAX base station. In this manner, the terminal adjusts the respective parameters of power, timing and frequency. However, the WiMAX base station cannot notify the WiMAX-embedded wireless terminal of the Ranging completion, that is, it cannot adjust a value of any of the parameters even by adjusting prescribed number of times. If the parameters cannot be adjusted, Network Entry fails, so that the WiMAX-embedded wireless terminal cannot get service in Down Link or Up Link.
One cause why respective parameters cannot be adjusted to be within permissible ranges in a CDMA Ranging Request by a prescribed number of times includes, for example, that the transmission power of the WiMAX-embedded wireless terminal is lower than the transmission power of the WiMAX base station.
A WiMAX-embedded wireless terminal is often used with information equipment such as a PC, or used independently as a mobile phone terminal. In either case, the WiMAX-embedded wireless terminal needs to be operated in a power-saving mode. As such, power consumption of the WiMAX-embedded wireless terminal even when operating at maximum power is lower than that of a WiMAX base station. The WiMAX-embedded wireless terminal cannot often gain a sufficient receiving electric field in Up Link, in which the WiMAX-embedded wireless terminal is a transmitter, even in an area where in Down Link, in which the WiMAX base station is a transmitter, the WiMAX-embedded wireless terminal can receive a signal in a preferable state in which it can gain a sufficient receiving electric field.
Another cause includes the following.
In a CDMA Ranging Request, timing is adjusted in addition to the power. Accordingly, in Up Link, when a WiMAX base station receives the signals transmitted from WiMAX-embedded wireless terminals, if the respective signals are received at different times, interference is caused among Subchannels, thereby causing a great influence on demodulation.
In Down Link, a WiMAX-embedded wireless terminal identifies the head of a Down Link Subframe in Preamble, so that a delay due to propagation causes no problem. On the other hand, in Up Link, a WiMAX base station defines the head of an Up Link Subframe by a TTG (Transmit/Receive Transition Gap) from the end of a Down Link Subframe. As such, if a delay due to propagation is too long, a WiMAX base station can not receive a CDMA Ranging Request.
To address the case in which Up Link communication is difficult or the communication quality is low, a technique is provided such as handover to a heterogeneous network such as a mobile network as disclosed in Japanese Patent Laid-Open No. 2005-184820, or amplification of a signal source by a relay as disclosed in Japanese Patent Laid-Open No. 2006-196985. However, communication by a WiMAX system for only a Down Link service has not been realized.
According to the invention disclosed in Japanese Patent Laid-Open No. 2005-184820, handover is performed to a heterogeneous network if a WiMAX Up Link service is unnecessary and only a Down Link service is necessary. Thus, if the fee for communication via the heterogeneous network is high, the user must pay a high communication fee.
The invention disclosed in Japanese Patent Laid-Open No. 2006-196985  has the problem in which there is a need to invest in equipment for installing a relay.
A technique for Down Link communication includes “Communication System” disclosed in Japanese Patent Laid-Open No. 2003-153238, or “Radio Communication System and Retransmission Control Method Thereof” disclosed in Japanese Patent Laid-Open No. 8-274758.
However, according to the inventions disclosed in Japanese Patent Laid-Open No. 2003-153238 and Japanese Patent Laid-Open No. 8-274758, a communication system for a Down Link service is not specified. Moreover, they do not assume a WiMAX system. Furthermore, the patent documents do not disclose anything about a connection method.
Further, a Down Link service is limited to a broadcast channel according to the invention disclosed in Japanese Patent Laid-Open No. 2003-153238, so that it is difficult to apply the invention to Down Link services other than the broadcast channel.
As in the above, according to the inventions disclosed in Japanese Patent Laid-Open No. 2003-153238 and Japanese Patent Laid-Open No. 8-274758, it is impossible to provide only a Down Link service by a WiMAX system.